1. Field of the Invention
The subject invention is generally related to a cell which is inserted in the fluid flow line in such a manner to permit non-invasive monitoring of the fluid and is specifically directed to an assembly permitting sensors to interface with the fluid through semi-permeable membranes thereby allowing for photochemical reaction which may in turn be optically monitored through windows provided in the sensors. This combination provides for non-invasive real time monitoring of various entities contained in the fluid.
2. Description of the Prior Art
In most medical applications where fluids are either being introduced into the body or withdrawn from the body, the purity of the fluid must be maintained. Where such fluids have to be pumped, monitored, or subjected to temperature and other environmental controls, these activities must be done in a non-invasive manner. Over the years, a widely used non-invasive monitoring technique has been developed where the fluid is exposed to a light emitting device and a sensor, wherein the reflected light spectrum as modified by the photochemical reactions between the sensor elements and certain specific constituents within the fluid is monitored to determine the presence or lack of specific constituents, as well as the concentration of the constituents. The practice has become a widely accepted method for monitoring and diagnosing the condition of human blood. This methodology is particularly useful during the conduct of Cardio Pulmonary Bypass during open heart surgery because the information is received by the technician immediately.
A well known device specifically directed to directing a flow of blood through a photochemical monitoring zone is the Cardiovascular Devices Inc. (CDI) flow through cell, models 6640, 6630 and 6620, generally shown and described in U.S. Pat. Nos. 4,460,820 and 4,786,474. As there shown, the cell includes a flow through body having a window opening covered by a membrane. Sensors are seated adjacent to the membrane and may monitor the blood passing through the cell. While this device has gained widespread commercial acceptance over the last decade, there are several drawbacks which make it cost prohibitive in certain applications and certainly increase the costs of treatment wherever it is used as part of the diagnostic regimen. This is particularly true since such units are disposable and costs associated with their use is repeated each time the technique is employed.
One of the problems driving up the cost of the CDI cell is the assembly design coupled with the labor intensive methods required for fabrication. As particularly described in U.S. Pat. No. 4,640,820, the flow through cell includes a membrane support and a pair of membranes for isolating the sensors from the flowing blood. The means for mounting the membranes in the window under the lens of the sensor includes a groove in the support structure for receiving the edge portions of the membrane. Typically, two different membranes are utilized, each having a different optical response and a different permeability factor, depending on the application. In order to mount the membranes in the window, the edge portion of the first membrane is extended into the groove in the structure separating the two windows and the second membrane is extended over the retaining means outwardly from the groove and then into the groove in order to form a smooth transition with the first membrane. Specifically, one of the membranes is wrapped partially around the retainer which is then inserted in the groove.
As described in the '820 patent, a region of the end portion of one membrane extends beyond the end portion of the second membrane within the groove and is adhered to the wall of the groove. This allows a portion of the first membrane to be adhered directly to the support even if only one face of the membrane is capable of being adhesively attached. It is this wrapping feature which establishes the proper firmness of the membranes in the assembly.
While effective for the purpose of joining the adjacent edges of the two membranes, the remaining three sides of each of the two membranes must also be sealed in a separate groove around the perimeter of the cell and the quality of this seal is impaired by the excess material resulting from the aforementioned wrapping technique. The complicated wrapping technique and placement steps as defined in the '820 patent has resulted in an increase in costs while at the same time reducing the reliability of the assembly due to both the skill required and the narrow tolerances which must be maintained to assure a proper fit of the membranes in the cell.
Therefore, there remains a need for a high quality, low cost cell capable of being used in the existing equipment as a disposable flow through cell for non-invasive monitoring of bodily fluids, especially blood.